In this study, spatial calibration of tracked ultrasound was compared by using a calibration phantom made of
LEGO&reg; bricks and two 3-D printed N-wire phantoms. METHODS: The accuracy and variance of calibrations were
compared under a variety of operating conditions. Twenty trials were performed using an electromagnetic tracking device
with a linear probe and three trials were performed using varied probes, varied tracking devices and the three
aforementioned phantoms. The accuracy and variance of spatial calibrations found through the standard deviation and
error of the 3-D image reprojection were used to compare the calibrations produced from the phantoms. RESULTS: This
study found no significant difference between the measured variables of the calibrations. The average standard deviation
of multiple 3-D image reprojections with the highest performing printed phantom and those from the phantom made of
LEGO&reg; bricks differed by 0.05 mm and the error of the reprojections differed by 0.13 mm. CONCLUSION: Given that
the phantom made of LEGO&reg; bricks is significantly less expensive, more readily available, and more easily modified than
precision-machined N-wire phantoms, it prompts to be a viable calibration tool especially for quick laboratory research
and proof of concept implementations of tracked ultrasound navigation.

PURPOSE: Spatial calibration of tracked ultrasound systems is commonly performed using precisely fabricated phantoms. Machining or 3D printing has relatively high cost and not easily available. Moreover, the possibilities for modifying the phantoms are very limited. Our goal was to find a method to construct a calibration phantom from affordable, widely available components, which can be built in short time, can be easily modified, and provides comparable accuracy to the existing solutions. METHODS: We designed an N-wire calibration phantom made of LEGO® bricks. To affirm the phantom’s reproducibility and build time, ten builds were done by first-time users. The phantoms were used for a tracked ultrasound calibration by an experienced user. The success of each user’s build was determined by the lowest root mean square (RMS) wire reprojection error of three calibrations. The accuracy and variance of calibrations were evaluated for the calibrations produced for various tracked ultrasound probes. The proposed model was compared to two of the currently available phantom models for both electromagnetic and optical tracking. RESULTS: The phantom was successfully built by all ten first-time users in an average time of 18.8 minutes. It cost approximately $10 CAD for the required LEGO® bricks and averaged a 0.69mm of error in the calibration reproducibility for ultrasound calibrations. It is one third the cost of similar 3D printed phantoms and takes much less time to build. The proposed phantom’s image reprojections were 0.13mm more erroneous than those of the highest performing current phantom model The average standard deviation of multiple 3D image reprojections differed by 0.05mm between the phantoms CONCLUSION: It was found that the phantom could be built in less time, was one third the cost, compared to similar 3D printed models. The proposed phantom was found to be capable of producing equivalent calibrations to 3D printed phantoms.

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